The Aspergillus nidulans rcoA gene is required for veA-dependent sexual development

The Aspergillus nidulans rcoADelta mutant exhibits growth and developmental defects. We show that the rcoADelta mutant lacks cleistothecia and is self-sterile. In crosses with wild-type strains, rcoADelta nuclei do not contribute to the cleistothecial walls. Furthermore, sexual development resulting from veA overexpression is rcoA dependent, indicating that rcoA lies downstream of veA in the sexual development pathway.     

A putative G protein-coupled receptor negatively controls sexual development in Aspergillus nidulans

G protein-coupled receptors (GPCRs) are key components of heterotrimeric G protein-mediated signalling pathways that detect environmental signals and confer rapid cellular responses. To broaden our understanding of signalling mechanisms in the filamentous fungus Aspergillus nidulans, intensive analyses of the Aspergillus nidulans genome have been carried out and nine genes (gprA approximately gprI) that are predicted to encode seven transmembrane spanning GPCRs have been identified. Six of nine putative GPCRs have been disrupted and the gprD gene was found to play a central role in coordinating hyphal growth and sexual development. Deletion of gprD (Delta gprD) causes extremely restricted hyphal growth, delayed conidial germination and uncontrolled activation of sexual development resulting in a small colony covered by sexual fruiting bodies. Genetic studies indicate that GprD may not signal through the FadA (G alpha)-protein kinase A (PKA) pathway. Elimination of sexual development rescues both growth and developmental abnormalities caused by Delta gprD, suggesting that the primary role of GprD is to negatively regulate sexual development. This is supported by the fact that environmental conditions inhibiting sexual development suppress growth defects of the Delta gprD mutant. We propose that the GprD-mediated signalling cascade negatively regulates sexual development, which is required for proper proliferation of A. nidulans.     

The chsA gene from Aspergillus nidulans is necessary for maximal conidiation

A fragment from the open reading frame of the cloned chsA gene from Aspergillus nidulans was deleted and replaced with the argB gene. The resulting construct was used to replace the wild-type chsA gene in an argB deletion strain. The growth and morphology of the vegetative hyphae from the resulting chsA disruptant strain were indistinguishable from those of a wild-type strain but the chitin content of the hyphae from the disruptant was reduced to approximately 90% of that of wild-type. The disruptant showed reduced ability to produce the asexual spores (conidia) that are formed by differentiated aerial hyphae called conidiophores. The ability to form undifferentiated aerial hyphae was not impaired in the disruptant. The conidiophores and conidia produced by the disruptant were indistinguishable from those of wild-type. Conidium formation by the disruptant grown on a variety of media was reduced to about 30% of the wild-type. A chsE null strain did not show a defect in conidiation but a strain in which both chsA and chsE were inactivated produced about 3% of the conidia of wild-type. That finding supports the hypothesis that chsA and chsE encode a partially redundant function necessary for conidiophore development.     

The orlA gene from Aspergillus nidulans encodes a trehalose-6-phosphate phosphatase necessary for normal growth and chitin synthesis at elevated temperatures

A cosmid carrying the orIA gene from Aspergillus nidulans was identified by complementation of an orlA1 mutant strain with DNA from the pKBY2 cosmid library. An orlA1 complementing fragment from the cosmid was sequenced. orlA encodes a predicted polypeptide of 227 amino acids (26 360 Da) that is homologous to a 211-amino-acid domain from the polypeptide encoded by the Saccharomyces cerevisiae TPS2 gene and to almost the entire Escherichia coli of otsB-encoded polypeptide. TPS2 and otsB each specify a trehalose-6-phosphate phosphatase, an enzyme that is necessary for trehalose synthesis. orlA disruptants accumulate trehalose-6-phosphate and have reduced trehalose-6-phosphatate phosphatase levels, indicating that the gene encodes a tre-halose-6-phosphatate phosphatase. Disruptants have a nearly-wild-type morphology at 32°C. When germinated at 42°C, the conidia and hyphae from disruptants are chitin deficient, swell excessively, and lyse. The lysis is almost completely remedied by osmotic stabilizers and is partially remedied by N-acetylglucosamine (GlcNAc). The activity of glutamine:fructose-6-phosphate amido-transferase (GFAT), the first enzyme unique to aminosugar synthesis, is reduced and is labile in orIA disruption strains. The findings are consistent with the hypothesis that trehalose-6-phosphate reduces the temperature stability of GFAT and other enzymes of chitin metabolism at elevated temperatures. The results extend to filamentous organisms the observation that mutations in fungal trehalose synthesis are highly pleiotropic and affect aspects of carbohydrate metabolism that are not directly related to trehalose synthesis.